This invention relates generally to electrical stimulation of a person's brain and in particular to an electrical stimulation system and method for treating tinnitus.
Tinnitus is a distressing symptom afflicting 10 to 15 percent of the general population. Twenty percent of those afflicted suffer from a significantly decreased quality of life; for example, many experience insomnia, depression, and other affective disorders.
When sound waves hit the eardrum, they are transmitted via the middle ear bones to the cochlea. In the cochlea, the sound waves are translated into neural patterns which are transferred via the cochlear nerve to the brainstem and then to the auditory cortex of the brain. The auditory system is organized in such a way that in the cochlea specific hair cells are activated by specific frequencies. This tonotopic organization is found throughout the auditory system. In particular, the auditory cortex is organized such that specific areas process the auditory information of specific frequencies.
Any lesion of the auditory tract that influences the normal function of the tract can generate tinnitus. In Meniere's disease, tinnitus is caused by a dysfunction of the cochlea; in acoustic neuroma, tinnitus is caused by a lesion of the vestibular nerve compressing the cochlear nerve; and in temporal brain tumor located in or near the auditory cortex, tinnitus is caused by direct interference with auditory cortex processing. The way in which tinnitus arises remains a matter of debate. However, some recent authors have suggested that tinnitus can be considered as an auditory phantom phenomenon similar to the phantom pain that arises after amputations.
Nerves and the brain are not hardwired, but are rather a constantly adapting network based on Darwinist principals. Nerve tracts are made up of nerve cells, each having a very specific function. Nerve cells that survive are those that are best suited for a specific function, which explains why humans have many more brain cells at birth than at the age of three years. Any change in a human's environment causes the brain to adapt to better process the changing incoming information. At birth, the auditory system is not completely hardwired and becomes functional only after it is exposed to environmental sounds. This exposition to sounds leads to a tonotopy that results from the death of synapses and brain cells that are not fit for a specific function, much as a sculpture is formed by removing the unnecessary marble from an original marble block. In the resulting tonotopy formed by exposure to sounds during the neonatal period, every brain cell processes a specific sound frequency only connected with those hair cells of the cochlea that process the same sound frequency.
When a lesion of the auditory tract occurs—for example, a lesion of the high frequencies caused by a sound trauma or the use of particular antibiotics—brain cells that process particular frequencies may become inactive. In order to prevent their death due to inactivity, the inactive cells grow into neighboring areas of the auditory cortex and thus begin to process other frequencies. For example, if a lesion of the high frequencies occurs, the brain cells may adapt to process middle frequencies in order to remain active. This process of cells altering their functions in accordance with changing demands is known as neural plasticity, or neuroplasticity. The mismatch between the genetically determined function of such cells and their active adapted processing leads to the conscious phenomenon referred to as tinnitus. This cortical reorganization can be demonstrated by a functional MRI (fMRI), PETscan, or magneto-encephalographic study.
It has been demonstrated that electrical stimulation of the auditory cortex in humans can reorganize the tonotopy of the cortex. External techniques have been used to electrically stimulate the auditory cortex in order to reorganize the tonotopy of the cortex to suppress the effects of tinnitus. For example, transcranial magnetic stimulation (TMS) devices have been used external to a person's head to stimulate the person's auditory cortex in order to provide temporary relief from the effects of tinnitus.